The ITU-T recommendation entitled The International Public Telecommunication Numbering Plan. E.164 specifies the current international Numbering Plan Administration (NPA) convention for the global switched telephone network (GSTN) and the national public switched telephone networks (PSTNs) that comprise it. E.164 specifies a maximum of 15 digits, although the number of digits required for calling within a nation varies. See also GSTN, NPA, and PSTN.